Fuel Problems & Port Fuel Injection
In many ways the problems caused by gasoline stem from the fact that fuel metering systems (LE-Jetronic and Motronic) are incredible precise when compared to carburetors. As a result, these systems are more sensitive too and easily affected by deposit formation.
In 1985, problems first arouse with Port Fuel Injection (PFI) fouling. Note, the BMW motorcycle LE-Jetronic and Motronic systems are both PFI. The assumption at the time was that gasoline additives which had been increasing in popularity, were the problem. However, the problem was more complex than just the additives themselves.
Research has shown that several conditions contribute to injector fouling and that some of these conditions are in fact more serious than the additives. Specifically, injector design, driving pattern, fuel seepage and temperature.
The pintle type injectors used in BMW's motorcycles are prone to deposit formation. The flow control pintle in this style injector is manufactured to very exact tolerance. The metering orifice is approximately 0.002 inch and the pintle provides a surface for deposits. Deposits themselves, form during the hot soak period immediately after the engine is shut off. Therefore, a motorcycle used primarily for short trips would be more susceptible to this type of deposit formation. Fuel leaking past the pintle during hot soak, will increase the rate at which deposits form. Increases in temperature, will also increase deposit formation.
Early on, manufacturers encouraged oil companies to include cleaners in their fuel for PFI. Most responded and the problem is not as severe today. However, the problem has not gone away. Note that as little as a 10% reduction in flow of any injector can result in drivability problems. These problems include uneven idle, reduced power, poor fuel economy, hard starting and even stalling.
In addition to injector deposits, we have to consider Intake Valve Deposits (IVD). Valve deposits have always been present in engines. In older engines, they were the result of oil, soft and gummy. Today's engines are built to tighter tolerances and the valves are exposed to less oil. In addition, PFI introduces fuel directly ahead of the intake valves. As a result, valve deposits are more fuel related and much harder.
IVD's affect the flow characteristics of the air/fuel mixture robbing power. They also absorb fuel on startup until they become saturated. As a result, these deposits can cause a lean condition during motorcycle warm up. As with injector deposits, short trips also contribute to this type of deposit formation.
Fuel composition and detergent chemistry play a key role in IVD formation. BMW developed the first test to rate fuels and their affect on IVD formation. The "BMW Cleanliness Test" is used by several companies to certify the cleanliness of their fuels. Work is currently under way to develop a comprehensive series of industry standard tests for fuel cleanliness certification.
Concerns about deposits have become so prevalent that some manufacturers have again developed recommended fuel lists. Using a gasoline with effective deposit control additives should keep intake valves clean under nearly all driving conditions. Most oil companies advertise the benefits of such fuels:
"passed the BMW unlimited mileage test" or
"keeps intake valves clean" or
"provide total induction system cleanliness."
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